Recently, a heterogeneous network (sometimes abbreviated “HetNet”) is proposed in which multiple types of radio base stations (macro base stations, pico base stations, femto base stations, remote radio heads, etc.) having different transmission powers (transmission capabilities), are deployed in a multi-layered way (for example, Non-patent Document 1).
In a heterogeneous network, base stations having a higher transmission power (transmission capability), e.g., macro base stations, are likely to be selected as the radio access points for user terminals at the stage of cell search or handover, in comparison with other base stations having a lower transmission power (transmission capability), e.g., pico base stations. Accordingly, it is assumed that connections of user terminals (mobile terminals) are concentrated at base stations with higher transmission power, and thus, there will be a tendency for excessive communication load at base stations with higher transmission power.
Accordingly, a technique called cell range expansion has been proposed. The cell range expansion is a technique to give an offset value (bias value) to the reception quality or reception power from the low-power radio base station, the reception quality or the reception power being an index for cell selection by the mobile terminal. The reception quality or the reception power from the low-power radio base station to which an offset value has been added (or added in the unit of dB) is compared with the reception quality or the reception power from the macro base station. As a result, the reception quality or the reception power from the low-power radio base station is likely to become better than the reception quality or the reception power from the macro base station. Consequently, since mobile terminals select to connect to the low-power radio base station than the macro base station, the cell range of the low-power radio base station is expanded, and it is likely that the communication load of the macro base station is reduced.
However, when the cell range of the low-power radio base station is expanded by the cell range expansion (CRE), the mobile terminal located at the edge of the cell of the low-power radio base station may be subject to high levels of interference by radio waves from the neighboring macro base station. Therefore, a technique called enhanced inter-cell interference coordination or enhanced inter-cell interference control, which is an extension of the inter-cell interference coordination or inter-cell interference control, has been proposed. This technique is abbreviated as eICIC. The eICIC is described, for example, in Non-patent Document 2.
The eICIC is classified into a frequency domain-based eICIC and a time domain-based eICIC. In either type, eICIC is a technique to limit resources available for a macro base station in order to prevent or minimize interference at mobile terminals connected with the low-power radio base station.
In the frequency domain-based eICIC, multiple frequency bands are prepared. First frequency bands are used for downlink transmission from the macro base station to mobile terminals connected with the macro base station and for downlink transmission from the low-power radio base stations to mobile terminals at the centers of the cells of the low-power radio base stations (e.g., mobile terminals connected with the low-power radio base stations without CRE). Second frequency bands are used for downlink transmission from the low-power radio base station to mobile terminals at the edges of the cells of the low-power radio base stations (e.g., mobile terminals connected with the low-power radio base stations by virtue of CRE), and are not used for downlink transmission from the macro base station. Thus, it is expected to prevent mobile terminals at the edges of the cells of the low-power radio base stations from being interfered by radio waves from the macro base station.
In the time domain-based eICIC, the macro base station and the low-power radio base station use the same frequency band, but different time units (for example, subframes) are used for different purposes. The low-power radio base station is capable of doing continuous downlink transmission. However, the macro base station can perform downlink transmission only intermittently. As a result, a period during which only the low-power radio base stations perform downlink transmission (protected subframe) and a period during which the macro base station and the low-power radio base stations perform downlink transmission (non-protected subframe) are repeated. The non-protected subframes are used for downlink transmission from the macro base station to mobile terminals connected with the macro base station and for downlink transmission from the low-power radio base stations to mobile terminals at the centers of the cells of the low-power radio base stations (e.g., mobile terminals connected with the low-power radio base stations without CRE). The protected subframes are used for downlink transmission from the low-power radio base station to mobile terminals at the edges of the cells of the low-power radio base stations (e.g., mobile terminals connected with the low-power radio base stations by virtue of CRE). Thus, it is expected to prevent mobile terminals at the edges of the cells of the low-power radio base stations from being interfered by radio waves from the macro base station.